This is an excerpt from Forme of Cury
(England, 1390)
The original source can be found at
the Project Gutenberg website
LUMBARD MUSTARD. XX.VII. V. Take Mustard seed and waishe it & drye it in an ovene, grynde it drye. farse it thurgh a farse. clarifie hony with wyne & vynegur & stere it wel togedrer and make it thikke ynowz. & whan þou wilt spende þerof make it tnynne with wyne.
Other versions of this recipe:
For lumbardus mustard (Liber cure cocorum [Sloane MS 1986])
Lumbard mustard (Fourme of Curye [Rylands MS 7])